The present invention relates generally to a wavelength conversion device and more specifically to a wavelength conversion device that changes the wavelength of laser light by means of a nonlinear optical medium such as lithium niobate, for example.
Laser light generators or sources can produce coherent light with sharp directionality at high intensity. As a result, such sources are used in a wide variety of fields, e.g., the measuring, medical and processing fields, and in chemical industry. Most such sources produce light of a specific wavelength, which wavelength specificity limits their practical application. To overcome this limitation, laser wavelength conversion systems have been designed, using a nonlinear optical medium such as lithium niobate (LiNbO3), for example. One system with particular practicality is the second harmonic generation (SHG) system, which can halve the wavelength of laser light, and the optical parametric oscillation (OPO) system, which can increase wavelength. These systems employ a resonator, either internal or external to the laser light source, having end mirrors between which a nonlinear optical medium is inserted. Typical examples of such systems will be described with reference to FIGS. 6-8.
FIG. 6 shows a laser light source 10 and a nonlinear optical medium 20 in a resonant system for changing laser light L1 oscillated in the system to laser light L2 with half the wavelength of light L1 by second-harmonic generation. A laser medium 1 uses a YAG rod containing a laser-active substance such as Nd ions which are excited with exciting light EL generated by an exciting light source 2, producing laser light L1 with a wavelength of 1.06 .mu.m, for example. The laser resonant system for the light includes a total-reflection mirror 3 and a partial-reflection mirror 4. The laser light L1 in this system is passed through the nonlinear optical medium 20 and changed to a laser light L2 with a wavelength 0.53 .mu.m, which is the second harmonic based on the original light L1. The converted light is removed through the partial mirror 4. The second harmonic is generated as a secondary polarization wave according to the nonlinear optical coefficient of the medium, from the electric field of the primary polarization, using the incident laser light L1 to the nonlinear optical medium 20 as the fundamental wave. Because the secondary polarization is smaller than the primary polarization but proportional to the square of the electric field, it is necessary to strengthen the electric field to obtain a strong second harmonic. To this end, the laser light L1 is contained in the resonant system so that the intensity applied to the nonlinear optical medium 20 can be increased.
Another prior-art example, shown in FIG. 7, is also of a second-harmonic generation system, wherein a resonator 30 with a pair of mirrors 31 and 32 is provided on the outer side of a laser light source 10, within which the nonlinear optical medium 20 is disposed to pass the laser light L1 in only one direction from the laser light source 10 to an external resonator 30 via an isolator 5. The two mirrors 31 and 32 have high reflectivity for the laser light L1 and confine the laser light L1 in the resonator (30) to intensify the fundamental wave traveling through the nonlinear optical medium 20. The distance between the two mirrors 31 and 32 is adjusted so that the phase of the light reflected at the mirror 31 is opposite to the phase of the light reflected at the mirror 32. In this condition, the external resonator 30 has high transmittance for the laser light L1. Using the laser light L1 confined in the resonator as the fundamental wave, the converted laser light L2, which is the second harmonic of the laser light L1, is generated in the nonlinear optical medium 20. Mirror 31 is highly reflective and mirror 32 is highly permeable for the light L2, for removal of the light L2 through the mirror 32.
With reference to FIGS. 6 and 7, if the refractive index of the nonlinear optical medium 20 for the laser light L1 is different from that for the laser light L2, the laser light L1 and the laser light L2 propagate with different velocities. Then, the laser light L2 is generated with different phase at different locations in the optical medium 20 depends on the location in the medium, leading to reciprocal cancellation and reduction of wavelength conversion efficiency. To raise the conversion efficiency, it is necessary to satisfy a phase-matching condition, n.sub.1 =n.sub.2, i.e., the refractive indices n.sub.1 and n.sub.2 for light L1 and light L2 in the nonlinear optical medium 20 are the same.
As the refractive index in an optical crystal depends on angle to the optical axis, the nonlinear optical medium 20 is cut so that this phase-matching condition is satisfied.
Another prior-art system, shown in FIG. 8, has a configuration similar to that in FIG. 7, wherein laser light L1 oscillating in the laser light source 10 through is changed through a nonlinear optical medium 20 in the external resonator 30, which is an optical parametric oscillator, to laser light L3 and L4, each with a wavelength longer than the wavelength of the laser light L1. This optical parametric oscillator also utilizes secondary polarization via the nonlinear optical medium 20. With .omega.1 denoting the angular frequency of the laser light L1, and with .omega.3 and .omega.4 denoting the respective angular frequencies of the converted laser light L3 and laser light L4, the condition .omega..sub.1 =.omega..sub.3 +.omega..sub.4 is satisfied.
Improving the conversion efficiency of the optical parametric oscillation also requires that the phase-matching conditions described above be satisfied, the nonlinear optical medium 20 being cut to satisfy the phase-matching condition n.sub.1 .omega..sub.1 =n.sub.3 .omega..sub.3 +n.sub.4 .omega..sub.4, where n.sub.3 and n.sub.4 are the respective refractive indices for the light L3 and the light L4. While this allows both of converted laser light L3 and laser light L4 to be generated, satisfying this phase-matching condition is difficult in practice. As a result, only one of them is generated in many cases.